Negative bias for audio frequency amplifiers



S. W. SEELEY NEGATIVE BIAS FOR AUDIO FREQUENCY AMPLIFIERS Filed Sept. 30, 1955 5f arf WJee/e BY 5, 6 WW ORN EY Patented Oct. 8, 1935 UNITED STATES PATENT OFFICE NEGATIVE BIAS FOR AUDIO FREQUENCY AMPLIFIERS Delaware Application September 30, 1933, Serial No. 691,577

Claims.

This invention relates to radio receiving sets and is particularly, though not exclusively, applicable to receiving sets of the superheterodyne type in which oscillations are inherently present.

5 An object of the invention is to produce a radio receiving set in which no separate battery or other source is necessary for producing the negative grid bias.

Other objects will appear in the following description taken in connection with the drawing in which the single figure of the drawing illustrates a radio receiving circuit embodying the principles of the invention.

Referring to the drawing, the signals may be received on an aerial I of any desired type. The primary coil 2 has one end connected to the aerial and the other end connected to the ground G.

The secondary coil 3 is tuned by the variable condenser 4. To accommodate the circuit to aerials of different constants an adjustable condenser 5 isbridged between the antenna and the high voltage end of the coil 3.

The secondary coil 3 is connected to the control electrode 9 of a pentode tube I0. A suppressor grid I4 is used in tube It to prevent secondary emission from the plate I5 from deleteriously affecting the operation of the tube. However, the invention is not limited to any particular type of tube and this type is given by way of example only. The screen grid l6 of tube I0 is connected through wires I8, [9, to an intermediate voltage terminal 20 of the plate supply, the voltage in this case being 67.5 volts. The negative end 2| of this voltage supply is connected by wire 22 to ground G. The cathode 23 of this tube I0 is also connected to the ground G. The plate l5 of the tube I0 is connected to one end of primary 24, the other end of the primary being connected through wires 25 and 26 and resistance 2'! to the high voltage terminal 28 of the plate supply which in this case is given as 180 volts by way of example only.

The secondary 29 coupled to primary 24 is connected at one end to the control electrode 30 of tube 3|, the other end of this coil is connected by wire 32 and coil 33 and high resistance 34 to terminal 35 of resistance 36. The terminal 3! of this resistance is connected to ground G. In this particular case I have secured good results .by

making the resistance 36 of 50,000 ohms and resistance 34 of 250,000 ohms. Condensers 38 and 39 serve as radio frequency bypass condensers and prevent passage of the direct current negative bias currents directly to ground. The existence of the negative bias potentials will be later referred to. Condenser 40 tunes the coil 29 to the radio frequency signal. Trimming condenser 41 may also be used to permit unicontrol of the various tuning condensers. The tuning condenser 40 is connected in shunt to the coil 29 through 6 ground G.

The grid coil 42 of oscillator tube 43 is coupled to the plate coil 44 which latter coil is tuned to the desired oscillator frequency by condenser 45. This condenser may have a trimming con- 10 denser 46 to permit it to be unicontrolled with the other variable condensers. One end of this condenser is connected-to the ground G and the other one to the high voltage end of the coil 44. This plate circuit coil 44 is connected through 15 wire 41 to terminal 48 of the plate supply which in this case has a potential of 112.5 volts. Plate coil 44 is coupled tothe coil 33 so as to introduce the oscillator frequency into the grid circuit of the first detector tube 3|. One end of coil 42 is 20 connected to the oscillator grid 49 and the other end to the biasing resistance 36, the operation of which will be described later.

The plate or anode 50 is connected to one end of primary coil 5| tuned to the beat frequency 25 by variable condenser 52,the other end of this plate coil being connected through wire 26 to the high voltage terminal 28. Secondary coil 53 tuned to the intermediate frequency by condenser 54 has one end connected to the control electrode 30 55 of intermediate frequency amplifier 56,' the other end of this coil 53 is connected through the high resistances 51 and 58 to ground G. The values of these resistances of course is not critical. I have secured good results with values of one- 35 7 half megohm and one-fourth megohm respec tively. The two ends of resistance 51 are'connected to ground G through condensers l3 and 59. Suppressor grid 60 in tube 56 is connected directly to the filament 6| and to ground G. The 40 screen grid 62 of this tube is connected through wire H) to the intermediate voltage terminal 20. Anode 63 of tube 56 is connected to one end of primary 64 tuned by condenser 65 to the intermediate frequency. The other end of coil 64 is 45 connected through wire 26 to the high voltage terminal 28. One end of the secondary coil 66 is connected to parallel anodes 61 of a diode-triode. tube 68. The other end of this secondary coil is connected to the resistance 58 through ground G. 50 The cathode 69 of the tube 68 has one end connected to the ground G. The control electrode 10 of tube 68 is connected by an adjustable connection H to the resistance 58 and to ground G through the radio frequency bypass condenser 12. 56

The anode I3 of the triode portion of tube 68 is connected through resistance M which in this case may be about 150,000 ohms and through wire 25 to the high potential terminal 28 of the plate 10 pling and blocking condenser I8. 7 This grid is) also connected through a high resistance 19, which may be one megohrn'fan'd wire 89 to the junction point of coil 33 and resistance 34 in the oscillator circuit. The anode BI of the audio 15 amplifying tube TI is connected to one end of primary 82, the other end of which is connected to the high potential terminal 28 of the plate supply. This primary is v coupled, preferably through an iron core, to secondary 83 of the push 20 pull amplifier tube M, 85. One end of this coil- 83 is connected to the control electrodes 85 of tube 84 and the other end is connected to control electrodes 81 of the other tube 85. The intermediate portion of the secondary 83 is connected 2:} to wire 88 to ground G and to one side of the filaments 89 of the push-pull amplifiers. V

The anodes 99 and 9t, of the push-pull amplifiers are connected to opposite ends of the primary 92 which is coupled through an iron core 303130 the secondary 93, the terminals of which are connected to the loud speaker coil 94. The intermediate portion of this primary 92 is connected through wire 95 to the high voltage terminal 23. A: variable resistance 96 which by way 35 of example, may be 50,000 ohms, has one end connected to the anode 99 and the otherend connected to the anode 9i through a condenser 91 which may be .05 microfarad. This circuit is a shunt around coil 92 and acts as a tone control.

40 v The control electrodes 85 and 87 of the tubes 94 and 85 have a large number of turns or meshes which may besecured conveniently by winding two grid coils properly spaced from each other and connected together so that the electron 45 stream must pass through both coils to reach the anodes.

All of the cathodes in the tubes are'shown as of the direct heated filament type by way of example and are connected in parallel to. the 50 cathode battery 98. In case a 2.5 air depolarizer cell is used for'the filament, supply switch 99 should be opened so as to incorporate resistance I invthe: filament circuit. This resistance has a value of approximately .5 ohm, though of course 55 this is not a necessary value. When a two volt.

storage battery is used for the filament supply switch 99 is closed so as to short-circuit resistance I 00. The positive terminal IOI of the cathode supply is connected through wire I02 to one end 60 0f each of the filaments. V The negative end of the battery 98 is connected through wire I03 and.

wire 22 to ground G and thence to the other ends of the filaments.

Switches I 04, I are coupled together to enable way of example may be 175,000 cycles. 'Theoscil lator'output is fed intothe grid circuit of the tube 3| and the resultant frequency is produced in the plate circuit of thetube 3|. The rectify:

ing action produces an intermediate frequency in the plate coil 5|. Condensers 40 and 52 tune the coils 29 and SI respectively to the signal and intermediate frequencies. The output of this intermediate frequency amplifying tube 56 is fed 5 into the diode or Fleming valve portion of tube 68 and the audio'frequencies extracted. The rectified currents in this second detector pass from the cathode 69 to the anode 67 through coil 66,

'back to the filament through the ground. The 10 grid Illof the triode portion of this tube 68 being 'adjustably connected to resistance 58, receives the audio frequency signal voltage. This voltage produces an amplified signal current in the circuit of plate I3. The potential of the diode plate 6! causes sufiicient current to flow through the diode circuit to produce an initial negative bias of about 1 volt in resistance 58. Thus an initial negative bias is applied to the grids 9 and 55 of the tubes I0 and 56. This bias increases with the signal strength so that an automatic volume control results. Resistance 5! and audio frequency condenser I3 prevent the audio frequency from affecting the grids of these tubes. I

Since grid E6 of tube 11 is connected by coupling condenser I8 to the plate I3 of tube 68 such grid receives the amplified audio voltages applied across the resistance 19 and these audio currents are still further amplified by the tube 11 and the push-pull tubes 84, 85 and then fed into the loud speaker coil 94 through the transformer 92, .93. As previously stated, the volume of the reproduction may be controlled by the adjustable tap II on resistance 58 in a well understood way. I

The increased mesh of the grids of the power tubes 84,85 bring the cut-off point of the plate voltage practically to zero grid voltage. On account of this and the push-pull arrangement no negative bias is required on these tubes.

t will be noted from the oscillator circuits of tube 43 that the-currents rectified in the grid cathode circuit of the tube produce a direct current potential drop in resistance 36, the positive terminal being attached to the ground. This di- 45 rect current potential drop varies with the frequency of the oscillator, the higher-the oscillations the greater the potential drop. The grid of the first detector tube 3I is connected to thenegative end 35 of resistance 36 and therefore this detector receives a bias that varies with the intensity of the oscillations as described and claimed in my copending application Serial No. 676,814, filed June 21', 1933.

The grid 16 of audio-tube]? receives-its negai tive bias by being connected through resistance I9 and resistance 34 tothe negative terminal 350i oscillator resistance 36 and thisis an important feature of my invention. The audio frequencies are filtered out by resistance 34 and condenser no 38 and thus they do not modulate the oscillator currents. I r

As I-have previously stated, I disclosed in my above mentioned copending application how one can advantageously obtain a variable detector bias that increases with the intensity of the oscillator but at that time "I was unaware that this same bias could. be successfully used in an audio stage. Since the drop in the oscillator resistance 35 varies-from seven volts tofifteen volts in circuit arrangements that I had used it was thought that such a variable bias. voltage could not be usedinan audio amplifier'stage, but I have found;

complete receiving circuit I am able to eliminate all battery sources formerly used for supplying the grid circuits of receiving sets. This enables one to produce a receiving set of lower cost for use on farms and other places where power supply is not available. The upkeep of such sets is also reduced by the elimination of the C battery.

In order to enable one to understand the principles of the invention a particular embodiment has been described and certain values and characteristics have been given. These are by Way of example only as the invention is in no way limited to any particular circuit values, nor to any particular type of vacuum tubes. This will be apparent to those skilled in the art and it will be understood that various modifications may be made without departing from the spirit of the invention.

Having described my invention, what I claim is:

1. In superheterodyne receiving 7 sets, signal receiving means, an oscillator differing in frequency from the signal by a desired intermediate frequency and connected with said means, a second detector, an audio frequency amplifier, means to connect the output of said detector to the audio frequency amplifier, and means to rectify the high frequency potential of said oscillator and introduce it as a negative bias into the grid circuit of the audio amplifier.

2. In superheterodyne receiving sets, a vacuum tube heterodyne oscillator having a resistance in its grid-filament circuit, a vacuum tube radio frequency amplifier connected to said oscillator, a vacuum tube audio frequency amplifier, means to connect the output of the radio frequency amplifier-to the input of the audio frequency amplifier, and means to negatively bias the control electrode of the audio frequency amplifier by the potential drop in said resistance.

3. In superheterodyne receiving sets, a vacuum tube heterodyne oscillator having a resistance in its grid-filament circuit, a vacuum tube radio frequency amplifier connected to said oscillator, a vacuum tube audio frequency amplifier, means to connect the output of the radio frequency amplifier to the input of the audio frequency amplifier, means to negatively bias the control electrode of the audio frequency amplifier by the potential drop in said resistance, and a radio frequency bypass condenser around said resistance.

4. In superheterodyne receiving sets, a vacuum tube heterodyne oscillator having a resistance in its grid-filament circuit, a vacuum tube radio frequency amplifier connected to said oscillator, a vacuum tube audio frequency amplifier, means to connect the output of the radio frequency amplifier to the input of the audio frequency amplifier, means to negatively bias the control electrode of the audio frequency amplifier by the potential drop in said resistance, and means to prevent the audio frequency of said audio amplifier from passing through said resistance.

5. In superheterodyne receiving sets, a vacuum tube heterodyne oscillator having a resistance in its grid-filament circuit, a vacuum tube radio frequency amplifier connected to said oscillator, a vacuum tube audio frequency amplifier connected to said radio frequency amplifier and having its grid connected through a resistance to the negative end of the first resistance and an audio frequency bypass condenser connected between the last mentioned grid and the other end of the first mentioned resistance to shunt audio frequencies therearound.

STUART W. SEELEY. 

